Anchoring devices of the type here concerned are components of systems that are used in surgery chiefly to stabilize posteriorly spinal column segments. They are designed either as pedicle screws, which are inserted through the pedicles into the vertebral body, or as hooks that are attached to the column by their blades. In either case, they are linked to longitudinal members (support rods or the like). By means of such systems, the vertebrae concerned are maintained at a distance from one another, or are compressed or maintained in traction. Depending on the design of the anchoring device, the connection to the longitudinal member may have several degrees of freedom, which can be fixed with a one or more elements.
The aim, as regards the use of pedicle screws, is to ensure a tension-free connection and an active anatomical repositioning. Depending on the design, the rotation of the pedicle screw in the plane defined by the longitudinal axes of the pedicle screws and the support rod can be blocked before, after or simultaneously with the sliding movement along the longitudinal member.
The aim, as regards the use of hooks, is to ensure a correct anatomical positioning of the hook. Depending on the design, the rotation of the hook can be blocked before, after or simultaneously with the sliding movement along the longitudinal member. The hooks are inserted or engaged, as necessary, cranially (lamina or transverse process) or caudally (pedicle or lamina).
The following requirements arise in the operating-room installation of such anchoring devices:
It must be possible to install the devices together with the rods in a manner that is simple and quick, in order to spare the patient avoidable operating time; PA0 In the installation, the anchoring components (pedicle screws or hooks) must exert no undesired forces or moments on the bearing bone parts. PA0 A connection between the pedicle screws (or hooks) and a support rod can be achieved simply and quickly, and PA0 Undesired stresses and moments when this connection is created can be prevented. PA0 Fixation of all degrees of freedom with a single element.
A large number of such anchoring devices exist, which aim at the two aforementioned qualities. Two of these solutions may be considered significant to the present invention.
DE-GM 8 915 443 describes an anchoring device in which a holder for the support rod, which holder is connected with the element that effects attachment to the bone, is U-shaped, and has an internal thread. A support rod is inserted into the U-shaped holder. A stop is then screwed into the internal thread against the rod. The underportion of the stop and/or the rod has a surface quality such as to ensure a firm connection between the stop, held by the internal thread, and the rod. This connection can be made very quickly during an operation. However, the rod can be tightened in only one direction, referred to the attachment element in the boney substance of the vertebra. If two attachment elements are therefore not very exactly aligned, something that is difficult to achieve, when the stops are tightened undesired stresses occur at both rod ends.
A device that constitutes an improvement over DE-GM 8 915 443 is described in EP-Al 0 300 881. This patent too involves a U-shaped holder in which the rod can be inserted after the pedicle screws have been positioned or the hooks have been engaged. This U-shaped holder has in the center of the vertex of the U a tip pointed toward the inside of the U, on which the rod comes to rest. At the top ends of the U a slider is positioned on the interior of the jaw. A part is inserted into this slider, which part has two screws that lie in front of and behind the tip on the axis of the rod and which can be turned against the axis. If the rod is resting on the two holder segments of the pedicle screws, by means of careful alternating turning of the four screws the rod can be attached to the pedicle screws without undesired stresses; however, this takes time and a certain skill in turning, if undesired stresses are to be prevented.